Wind turbine generator systems generating electricity using wind power, which is natural energy, have been conventionally known. This type of wind turbine generator system includes a rotor head to which wind turbine blades are attached, a main shaft coupled to the rotor head so as to integrally rotate with the rotor head, a gear box coupled to the main shaft that rotates by means of wind power received by the wind turbine blades, and a generator driven by shaft output power from the gear box, which are provided in a nacelle mounted atop a tower. In the thus-configured wind turbine generator system, the rotation of the main shaft and the rotor head provided with the wind turbine blades that convert wind power to rotary power generates shaft output power. The output power of the shaft, whose rotational speed is stepped-up by the gear box coupled to the main shaft, is transmitted to the generator. Accordingly, the shaft output power obtained by converting wind power to rotary power works as the driving source of the generator, and thereby electricity can be generated using wind power as the motive energy of the generator.
The above-mentioned wind turbine generator system has a problem in that the wind turbine blades are struck by lightning at the ends, intermediate portions, and other portions. Accordingly, in conventional wind turbine blades, a lightning protection system is employed in which metal pieces (so-called receptors) for lightning conduction are attached to the wind turbine blades and the metal lightning-conducting portion receives lightning energy and discharges the energy to the earth.
In addition, there are also lightning protection systems in which electrically conductive metal sheets for lightning conduction are mounted on surfaces of wind turbine blades and the metal sheets for lightning conduction receive lightning energy and discharge the energy to the earth (for example, see Patent Document 1).    Patent Document 1: U.S. Pat. No. 7,040,864